428 BOTANICAL GAZETTE [December 



On one tracheid wall (in this case the one beneath) two separate 

 pits have formed, each with its own pore; while on the adjacent 

 wall of the next tracheid, one large, somewhat scalariform pit has 

 been retained. Just below this comes a pair of completely separate 

 pits, from their shape and approximation evidently formed by 

 the division of what potentially was a single long one. Then 

 come two more pits of the fusion type, after which the regular type 

 of pitting is resumed. Such examples are often found at the ends 

 of tracheids, where, as noted later, there are other primitive 

 characters. 



most 



pits in Cycads. 



approximate 



as to appear slightly flattened. This is the typical condition in the 

 Cordaiteae as described by Scott, and has generally been con- 

 sidered the most primitive bordered condition, although more 

 specialized than the reticulate type. The outline of the pits in 

 the Cycads, however, is more often curved. 



In the Cycads the scattered type of pitting is also found, origi- 

 nating by the elimination of pits from portions of the tracheid. 

 Fig. 6 from a radial section of a Dioon spinulosum stem shows this 

 feature. In the lower part of the right hand tracheid we have 

 biseriate pitting covering the radial wall, with here and there a 

 pit obliterated. The position of the vanished pits is indicated by 

 lighter areas caused by the thinning of the primary wall. These 

 are the primordial pits of Sanio, which have formed as if bordered 

 pits were to be located over them as usual. Farther toward the 



ement 



common 



some 



eliminated 



thinning of the primary wall being visible here and there. This 

 scattered pitting is seen also in other tracheids of the figure, the 

 primary pit areas being very evident, especially just above and 

 below the center of the second tracheid from the left. At the 

 center is an interesting small bordered pit surrounded by a slightly 



boundary 



This 



seems like a case of partial elimination. The irregular obliteration 

 of the pits has left in some places isolated groups of pits like those 



